This invention relates to a metal ring binder having auxiliary springs. Like many ring binder mechanisms, intended for notebooks, the binder described below comprises three rings, each made in two halves. The lower end of each spring half is swaged or otherwise connected to one of two blades which are held in compression, edge-to-edge, within a curved sheet metal spine. The blades are stable at two positions: one, where the rings are closed, and another, where the rings are fully open. The compression force exerted on the blades by the spine cause a toggling action--so that the blades are unstable at intermediate positions--and the mechanism proceeds to either extreme position with a familiar snap when one applies enough opening or closing force to the rings. A lever mechanism is usually installed at one or both ends of the spine, to assist one in opening the rings, or in locking them closed.
In most ring binders, the spine provides the only spring force for the rings, flattening somewhat as the blades pass their intermediate (coplanar) position. It can be shown that the bending stress is distributed throughout the length of the spine, but that greater bending stress occurs near the rings. The designer must therefore select metal for the spine having sufficient yield strength, modulus, and thickness in the vicinity of the rings, to provide adequate spring action and to survive repeated opening and closing. As a consequence, when using sheet metal of uniform thickness, the spine may be substantially thicker than necessary away from the rings. Additionally, the designer may have to compromise his choice of spine material to provide the needed clamping force on the edges of the blades. In some ways, therefore, it would be better to supplement the spring action with elements other than the spine metal.